When an x-ray image is obtained, there is generally an optimal distance and angle between the radiation source and the two dimensional receiver that records the image data. In most cases, it is preferred that the x-ray source provide radiation in a direction that is perpendicular to the surface of the recording medium. For this reason, large-scale radiography systems mount the radiation head and the recording medium holder at a specific angle relative to each other. Orienting the head and the receiver typically requires a mounting arm of substantial size, extending outward well beyond the full distance between these two components. With such large-scale systems, source-to-image distance (SID) is tightly controlled and unwanted tilt or skew of the receiver is thus prevented by the hardware of the imaging system itself. Further, because the spatial positioning and geometry of conventional large-scale systems is well-controlled, proper use and alignment of a grid, positioned in front of the imaging receiver, is straightforward.
Mobile x-ray apparatus are of particular value in intensive care unit (ICU) and other environments where timely acquisition of a radiographic image is important. Because it can be wheeled around the ICU or other area and brought directly to the patient's bedside, a mobile x-ray apparatus allows an attending physician or clinician to have recent information on the condition of a patient and helps to reduce the risks entailed in moving patients to stationary equipment in the radiological facility.
The perspective view of FIG. 1 shows an example of a conventional mobile x-ray apparatus that can be employed for computed radiography (CR) and/or digital radiography (DR). A mobile radiography unit 600 has a frame 620 that includes a display 610 for display of obtained images and related data and a control panel 612 that allows functions such as storing, transmitting, modifying, and printing of the obtained image.
For mobility, unit 600 has one or more wheels 615 and one or more handle grips 625, typically provided at waist-, arm-, or hand-level, that help to guide unit 600 to its intended location. A self-contained battery pack typically provides source power, eliminating the need for operation near a power outlet.
Mounted to frame 620 is a support member 635 that supports an x-ray source 640, also termed an x-ray tube or tube head, mounted on a boom apparatus 70, more simply termed a boom 70. A generator may also be mounted adjacent the tube head or, alternately, within frame 620. In the embodiment shown, support member 635 has a vertical column 64 of fixed height. Boom 70 extends outward a variable distance from support member 635 and rides up and down column 64 to the desired height for obtaining the image. Boom 70 may extend outward by a fixed distance or may be extendible over a variable distance. Height settings for the x-ray source 640 can range from low height for imaging feet and lower extremities to shoulder height and above for imaging the upper body portions of patients in various positions. In other conventional embodiments, the support member for the x-ray source is not a fixed column, but is rather an articulated member that bends at a joint mechanism to allow movement of the x-ray source over a range of vertical and horizontal positions.
With the advent of portable radiation imaging apparatus, such as those used in Intensive Care Unit (ICU) environments, a fixed angular relationship between the radiation source and two-dimensional radiation receiver and any accompanying grid is no longer imposed by the mounting hardware of the system itself. Instead, an operator is required to aim the radiation source toward the receiver surface, providing as perpendicular an orientation as possible, typically using a visual assessment. In computed radiography (CR) systems, the two-dimensional image-sensing device itself is a portable cassette that stores the readable imaging medium. In direct digital radiography (DR) systems, the two-dimensional image-sensing receiver is a digital detector with either flat, rigid, or flexible substrate support.
The receiver itself, however, may not be visible to the technician once it is positioned behind the patient. This complicates the alignment task for portable systems, requiring some method for measuring SID, tilt angle, and centering, and making it more difficult to use a grid effectively for reducing the effects of scatter. Because of this added complexity with a portable radiography system, the technician may choose not to use a grid; the result without a grid, however, is typically a lower-quality image.
There have been a number of approaches to the problem of providing methods and tools to assist operator adjustment of x-ray source-to-receiver angle. One conventional approach has been to provide mechanical alignment in a more compact fashion, such as that described in U.S. Pat. No. 4,752,948 entitled “Mobile Radiography Alignment Device” to MacMahon. A platform is provided with a pivotable standard for maintaining alignment between an imaging cassette and radiation source. However, complex mechanical solutions of this type tend to reduce the overall flexibility and portability of these x-ray systems. Another type of approach, such as that proposed in U.S. Pat. No. 6,422,750 entitled “Digital X-ray Imager Alignment Method” to Kwasnick et al. uses an initial low-exposure pulse for detecting the alignment grid; however, this method would not be suitable for portable imaging conditions where the receiver must be aligned after it is fitted behind the patient.
Other approaches project a light beam from the radiation source to the receiver in order to achieve alignment between the two. Examples of this approach include U.S. Pat. No. 5,388,143 entitled “Alignment Method for Radiography and Radiography Apparatus Incorporating Same” and No. 5,241,578 entitled “Optical Grid Alignment System for Portable Radiography and Portable Radiography Apparatus Incorporating Same”, both to MacMahon. Similarly, U.S. Pat. No. 6,154,522 entitled “Method, System and Apparatus for Aiming a Device Emitting Radiant Beam” to Cumings describes the use of a reflected laser beam for alignment of the radiation target. However, the solutions that have been presented using light to align the film or CR cassette or DR receiver are constrained by a number of factors. The '143 and '578 MacMahon disclosures require that a fixed Source-to-Image Distance (SID) be determined beforehand, then apply triangulation with this fixed SID value. Changing the SID requires a number of adjustments to the triangulation settings. This arrangement is less than desirable for portable imaging systems that allow a variable SID. Devices using lasers, such as that described in the '522 Cumings disclosure, in some cases can require much more precision in making adjustments than is necessary.
Other examples in which light is projected from the radiation source onto the receiver are given in U.S. Pat. No. 4,836,671 entitled “Locating Device” to Bautista and U.S. Pat. No. 4,246,486 entitled “X-ray Photography Device” to Madsen. Both the Bautista '671 and Madsen '486 approaches use multiple light sources that are projected from the radiation source and intersect in various ways on the receiver.
Significantly, the solutions noted above are often of little of no value where the receiver and its accompanying grid are hidden from view, lying fully behind the patient as may be the case, for example, for chest x-ray imaging with a portable system. Today's portable radiation imaging devices allow considerable flexibility for placement of the film cassette, CR cassette, or Digital Radiography DR receiver by the radiology technician. The patient need not be in a horizontal position for imaging, but may be at any angle, depending on the type of image that is needed and on the ability to move the patient for the x-ray examination. The technician can manually adjust the position of both the cassette or receiver and the radiation source independently for each imaging session. Thus, it can be appreciated that an alignment apparatus for obtaining the desired angle between the radiation source and the grid and image receiver must be able to adapt to whatever orientation is best suited for obtaining the image. Tilt sensing, as has been conventionally applied and as is used in the device described in U.S. Pat. No. 7,156,553 entitled “Portable Radiation Imaging System and a Radiation Image Detection Device Equipped with an Angular Signal Output Means” to Tanaka et al. and elsewhere, does not provide sufficient information on cassette-to-radiation source orientation, except in the single case where the cassette lies level. More complex position sensing devices can be used, but can be subject to sampling and accumulated rounding errors that can grow worse over time, requiring frequent resynchronization.
Thus, it is apparent that conventional alignment solutions may be workable for specific types of systems and environments; however, considerable room for improvement remains. Portable radiography apparatus must be compact and lightweight, which makes the mechanical alignment approach such as that given in the '948 MacMahon disclosure less than desirable. The constraint to direct line-of-sight alignment reduces the applicability of many types of reflected light based methods to a limited range of imaging situations. The complex sensor and motion control interaction required by the Tanaka et al. '553 solution would add considerable expense, complexity, weight, and size to existing designs, with limited benefits. Many less expensive portable radiation imaging units do not have the control logic and motion coordination components that are needed in order to achieve the necessary adjustment. None of these approaches gives the operator the needed information for making a manual adjustment that is in the right direction for correcting misalignment, particularly where a grid is used.
Yet another problem not addressed by many of the above solutions relates to the actual working practices of radiologists and radiological technicians. A requirement for perpendicular delivery of radiation, given particular emphasis in the Tanaka et al. '553 application, is not used in all cases because it is not optimal for all types of imaging. In fact, there are some types of diagnostic images for which an oblique (non-perpendicular) incident radiation angle is most desirable, provided that the grid alignment is acceptable for the given angle. For example, for the standard chest anterior-posterior (AP) view, the recommended central ray angle is oblique from the perpendicular (normal) by approximately 3-5 degrees. Conventional alignment systems, while they provide for normal incidence of the central ray, do not adapt to assist the technician for adjusting to an oblique angle.
Still other problems relate to the need to achieve a source-to-image distance (SID) that is well-suited for the image to be obtained and for the grid used. Conventional alignment solutions do not provide SID information, leaving it to the technician to make separate measurements or to make an approximate SID adjustment. Moreover, conventional solutions do not provide the technician with tools to help reduce backscatter, caused by misalignment or poor adjustment of the collimator blades. This type of scatter, while not particularly problematic with other types of radiographic imaging, such as dental and mammographic imaging, can be troublesome with portable radiographic imaging apparatus, since the radiation is directed over a broad area. Radiation that works past the imaging receiver and any blocking element associated with the receiver can inadvertently be reflected back into the receiver, adversely affecting image quality. To reduce backscatter as much as possible for chest x-rays and other types of x-ray, the technician is required to estimate the location and orientation or outline of the imaging receiver and to adjust the collimator accordingly.
Thus, it can be seen that there is a need for an apparatus that enables proper angular alignment and centering of a radiation source relative to an image receiver for recording a radiation image.